Managing Fruit Trees and Horticultural Crops During Prolonged Exposure to Subfreezing Temperatures

I know many of us enjoyed our warmer than average March with no snow, however, that has come around to be quite problematic. That kind of weather in March encouraged rapid progression of our flowers, fruit trees and other plant species, which had given us a spectacular show of spring blooms and flowers. The plants responded to the above average number of growing degree days that they accumulated and began their flowering, budding, and progressing through developmental stages and reproductive processes. The flowers and buds are the most important part of the plant when we consider fruit production. They are also the most delicate and vulnerable part of the plant.

So, what can be done to protect your plants? The challenge here is that our forecasted low is 25° F, with almost 12 hours forecasted to be below freezing and 6-8 hours forecasted to be in the 20’s. This is not just a light frost, but rather a hard freeze. There are a few steps that you can take to try to protect the plants, and some of these steps can be better utilized on a small scale, while others are better utilized on a larger scale.

On a smaller scale, simple actions like covering the plants with a sheet or some kind of fabric can help insulate them. This helps trap some of the warmer air from the ground around the plant and keeps the plant from being directly exposed to the colder air. It may also be of interest to protect the crown of the plant. This freeze may be severe enough to damage the foliage, but you can take steps to protect the crown and the roots by mulching with straw, leaves or wood mulch around the plant to keep the warmer air from the ground around these vital parts of the plant to keep it alive. Basically anything that you can do to insulate the plant, conserve the heat in the ground, and shield it from the cold air, will be beneficial to the plant and give it a better shot at making it through the night.

Adding a strand of Christmas lights (not LED) under the sheets covering your plants can help add a little extra heat to keep the ambient temperature a little warmer. Make sure that the lights are not touching the covering material and keep the covering material off of the plants. If the covering material is sitting directly on the foliage, that is defeating the purpose of having it there in the first place. Use stakes to keep the sheets or other covering materials just above the foliage. This will prevent the cold temperatures from being conducted onto the plants.

Watering the soil can also keep the immediate area around the plant a few degrees warmer. A wet soil is going to hold heat better than a dry soil. Even a few degrees can make a big difference. With single plants or with small enough shrubs or tree seedlings, an overturned bucket serves the save purpose as the sheets or coverings. Just remember, as soon as the sun comes up the next morning, you will want to remove whatever coverings you have to allow the plant to be exposed to the sunlight and begin to warm back up.

If you have a home orchard, it can be challenging to work with due to the size of the trees. One option is to hang the trees with the old-fashioned Christmas lights, however these are getting harder to find. They need to be the big bulb type, the newer LED Christmas-tree lights won’t work since they’re cool burning and won’t give off sufficient heat. You can improve the protection by also covering with a blanket or tarp.

Another option is to turn a sprinkler on your trees just as the freeze begins, to coat them with ice. Although it seems counterintuitive, the ice will protect the tree because the temperature beneath the ice will not drop below 32 degrees. For this method to work you will need to keep the sprinkler on until the temperatures rise above freezing. If you plan to use this method of frost protection and the cold temperatures last for a while, monitor to ensure that you are not creating a flood somewhere else.

On a larger scale, orchard growers and those with small fruit have big challenges. They are dealing with a lot of plants and trees over a big area. Some orchards use frost fans to try and protect the crop. Frost fans work by utilizing warmer/drier air from the ‘inversion’ layer to create air movement at the fruiting/flowering height in orchards on still cold nights—preventing damage to flowers, soft tissue, and fruit. Some orchards utilize water sprinkler systems to actually create a layer of ice on the trees. The key to using water is to continually use it to form clear ice. Clear ice means that an endothermic reaction is taking place and the warmth of the plant is being trapped inside it. If the ice starts to become cloudy, the plant is losing heat and damage can occur. Spraying water must continue the entire time the freeze event is taking place, and the sprays must keep going from before there is a freeze event that would damage the fruit until the ice is completely melted from the tree after the event. If the water stops spraying on the clear ice, it goes from being endothermic to exothermic, and the heat loss and ice will damage the fruit.

In some cases, burn barrels and smudge pots are placed around an orchard to add heat in the hopes of keeping the temperatures at, or above freezing. Unfortunately, it can take roughly 30-50 heaters per acre to effectively protect the trees. In some cases, if you are working with just a few trees, a strategically placed burn barrel can do the trick. Keep in mind that if you decide to use any type of heater you need to be extremely cautious using them.

Minimizing damage and losses requires knowledge of weather conditions and how to mitigate weather extremes. Just as important is having a knowledge of plant hardiness. Some plants will do just fine with this cold weather. Others may appear to be killed off but may regrow from the roots or the crown, even though the foliage appears dead. The important thing here is to not be over-reactive. Give the plants time to recover. Plants can be remarkably resilient. If you see signs of frost damage, do not prune off the affected parts or dig up the plant immediately. Wait until the weather warms up to see whether new leaves sprout. You may see healthy new growth at the base of the plant, at which point you can prune out the damaged parts.

Hoping for the best and doing nothing will not result in any positive outcomes, so take this opportunity to at least cover the plants that you can and give them a fighting chance to make it through the cold.

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